Over the last decades global energy consumption has increased exponentially and no end can be seen for the increased demand. Whereas exploitation of fossil fuels was previously focused on onshore fields, the limited amount of oil started serious efforts to find and exploit offshore gas and oil fields. Presently the state of the art for production from offshore fields is by use of fixed or floating manned platforms, and by tie-in of subsea production templates with subsea wells to these platforms. In some cases production is routed directly to an onshore receiving facility without a platform. In order to maintain a sufficiently high production from subsea satellites to a central platform or directly to shore, pressure boosting can be provided by using a multiphase pump or by separation followed by pumping and compression. Pumps have also been installed at seabed for direct seawater injection into the reservoir for pressure support for enhanced oil production.                There are several advantages that motivate for subsea location of pumps and compressor stations compared to location on platforms:        Safety for people by not working and living on platform: and not being transported by helicopters to and from        No risk of fire and explosion        No risk for blow-out from production risers up from seabed to platform and from platform to seabed        Security against sabotage        Cost saving both for capital and operation, i.e. reduced production cost for oil and gas        Increased production because the suction effect of compressors and pumps is closer to the wellheads        The equipment has stable ambient conditions, i.e. almost constant, cold temperature and almost constant, low flow seawater current velocity around the equipment and no waves, while the temperature at platforms can vary from e.g. −20° C. to +30° C. and the wind velocity can be at hurricane strength combined with extremely high waves.        The cold seawater can be utilized for cooling of motors and other electric and electronic equipment and process fluids        No visual pollution        Considerably lower weight and thereby lower material and energy amount for fabrication of a subsea plant        Lower carbon dioxide, i.e. climate gas emission for fabrication due to less material amount        Less carbon dioxide emissions during operation due to elimination of helicopter transport and operation of platform        Less carbon dioxide emission compared to platforms due to electric motors for running compressors and pumps and supply of electric power from shore or platform        Less energy consumption and climate gas emission per weight unit of oil and gas        
The disadvantage for subsea compressors per 2010 is that none has been installed and operated subsea, i.e. the technology is not proven. However, this is just a question of time, and the first subsea compressor station will probably be in operation in 2015 or earlier due to the strong motivation for this application.
Subsea pressure boosting is a recent technology. Subsea pressure boosting requiring a significant subsea step out length is a very recent technology using modern equipment and facing problems that are not met or is irrelevant elsewhere.
State of the art technology is defined in patent publication WO 2009/015670 prescribing use of a first converter arrangement in the near end, the topsides or onshore end, of a subsea step out cable and a second converter arrangement in the far end, the subsea remote end, of the subsea step out cable. A variable speed drive, VSD, is prescribed in either end of the step out cable. Subsea variable speed drives (VSD) for electric motors is also called variable frequency drive (VFD) and frequency converters or just converters and they represents state of the art technology. Neither in WO 2009/015670 or other publications is the Ferranti effect mentioned, nor is any problems associated with subsea VSDs discussed or indicated.
So far only a few subsea pumps and no subsea compressors are in operation. Subsea compression stations are however being developed and the first expected to be installed and in operation within some few years. Currently, subsea pumps and compressors are all driven by asynchronous motors. The step-out distance of installed pumps is not more that about 30 km from platform or shore and so far the depths are not below 1800 m. It is known that serious studies and projects are conducted by the oil industry aiming at installation of compressors at a step-out distance in the range of 40 to 150 km and at water depth down to 3000 m or more.
A realistic motor power is from about 200 kW for small pumps and up to 15 MW for compressors and in the future even larger motors can be foreseen. Subsea motors that are presently installed are supplied with power via AC (alternating current) cables from the location of the power supply, i.e. platform or shore, and in case of several motors each motor has its own cable and frequency converter (Variable Speed Drive, VSD, sometimes termed Adjustable Speed Drives ASD or Variable Frequency Drive VFD) at the near end of the cable in order to control the speed of each individual motor at the far end of the cable, ref. FIG. 1 and Table 2.
In the context of this patent description near end means the end of the power transmission near to the power supply. In subsea applications this is topsides platform location or onshore. Correspondingly, the far end refers to the other end of the transmission line close to the power loads, typically motor loads. Far end is not necessarily restricted to the high-voltage end of the transmission line. The term can be extended to busses or terminals of lower voltage which are part of the far end station such as e.g. a common subsea bus on the low-voltage side of a subsea transformer.
Compressors and pumps are often operated at maximum speeds between 4000 to 14000 rpm and 2000 to 5000 rpm, respectively. Thus the driving electrical motor has to have a rated speed in the order 2000 to 14000 rpm when using modern high speed motors without a gearbox between the motor and the pump or compressor. This mechanical speed corresponds to an electrical frequency range for the feeding drive of about 30 to 230 Hz for the example of a two-pole motor. Motors with more pole pairs would allow for lower maximum mechanical speed for the same electrical frequencies.
FIG. 1 illustrates the only solution so far used for transmission of electric power to installed pumps, in some cases without transformers between VSD and subsea motors, and this is referred to as First solution. This solution with one transmission cable per motor has the disadvantage of becoming expensive for long step-out; say more than 50 km, due to high cable cost.
A serious technical obstacle against this solution is that at a certain subsea step-out length, the transmission of electric power from a near end power source to a far end distant motor is not feasible because the transmission system will become electrically unstable and inoperable due to the Ferranti effect that later will be described. The invention will resolve this problem of instability.
FIG. 2 illustrates a solution that has been proposed for transmission of electric power to several loads at long step-out, Solution Two. This solution with one common transmission cable and a subsea power distribution system including one subsea VSD (Variable Speed Drive) per motor, will considerably reduce the cable cost for transmission, and also prevent the problem of electric instability by limiting the frequency of the current in the transmission cable to say 50-10 Hz, and the skin effect is also acceptable for such frequencies. The frequency is then increased by a VSD to suit the speed of the motor connected to the VSD. The Second Solution has however also disadvantages. These are expensive VSDs which are not proven for subsea use, and because such VSDs are composed of many electric and electronic components included a control system, they are susceptible to contribute to an increased failure rate of the electric transmission and subsea distribution system.
In the following will be described the inherent electrical problems of the existing First Solution (FIG. 1), with one motor at the far end of a long cable, and a Third Solution illustrated in FIG. 3 with several motors at the far end of a common long transmission and a common VSD at the near end.
For a long step-out distance from the power supply to the load, in the order of 50 km and above, the influence of the subsea cable is so strong that such a system has not been built yet for a limited load such as a single motor. The line inductance and resistance involve a large voltage drop from the power supply to the load. It is known that such a voltage drop is self-amplifying and can result in zero voltage at the far end. The longer the step-out distance the higher the transmission voltage has to be in order to reduce the voltage drop along the transmission line. However, a cable has a high capacitance and a long AC (alternating current) cable will exhibit significant so-called Ferranti effect. The Ferranti effect is a known phenomenon where the capacitive charging current of the line or cable increases with the line length and the voltage level. At a step-out length of 100 km the charging current in a cable can be higher than the load current, which makes it difficult to justify such an ineffective transmission system. A mare critical result is that the no-load voltage will be about 50% higher than the near end supply
Voltage that would destroy the cable and the far end transformer and connections. At a sudden load drop the far end voltage will jump to this high level. In addition there will be a transient peak of e.g. 50% giving like 100% in total, see Table 1 below where values marked with fat italic letters are above the voltage class margin of the insulation.
Today's systems with step-out distances in the order 30 km have not this problem, because the subsea step-out length and electric load in combination is still feasible.
TABLE 1Voltage rise at load trips due to Ferranti effect in different systemsMax. transmissionFar end transientfrequency fmax andStep-outStandardSource voltageFull-load andvoltage peak up afterFar end shaft powermotor speed ωmaxlengthcableat near end Uno-load voltage Ufull-load tripPump60Hz40km95mm220kV18.3 kV20.9 kV2.5 MW(3600rpm)30(36)kV20.2 kVFirst SolutionCompressor180Hz40km150mm232kV29.2 kV41.0 kV7.5 MW(10800rpm)30(36)kV34.8 kVFirst solutionsPump60Hz100km150mm226kV23.6 kV28.9 kV2.5 MW(3600rpm)30(36)kV27.5 kVFirst SolutionCompressor180Hz100km150mm228.5kV28.8 kV68.4 kV7.5 MW(10800rpm)30(36)kV52.7 kVFirst SolutionThree180Hz100km400mm245.6kV45.6 kV 155 kVcompressors andCompressor:45(54)kVunstablethree pumps.10800 rpmTotal 30 MWPump:Third solution5400 rpm
The Ferranti effect and skin effect—some considerations:
The Ferranti effect is a rise in voltage occurring at the far end of a long transmission line, relative to the voltage at the near end, which occurs when the line is charged but there is a very light load or the load is disconnected. This effect is due to the voltage drop across the line inductance (due to charging current) being in phase with the sending end voltages. Therefore both capacitance and inductance are responsible for producing this phenomenon. The Ferranti effect will be more pronounced the longer the line and the higher the voltage applied. The relative voltage rise is proportional to the square of the line length.
Due to high capacitance, the Ferranti effect is much more pronounced in underground and subsea cables, even in short lengths, compared to air suspended transmission lines.
A proposed equation to determine the Ferranti effect for a given system is:vf=vn(1+ω×C×L×I2)Where:vf=far end voltagevn=near end voltageω=2×3.14×ff=frequencyC=line capacitanceL=line inductanceI=line lengthI2=line length square
In the literature can also be found other expressions for the Ferranti effect, but in any cases it is agreed that the effect increases with transmission frequency, cable capacitance, length of cable and voltage.
From the above equation can be concluded that the Ferranti effect of a long line can be compensated by a suitable reduction of the electric frequency. This is the reason for the Second Solution with subsea VSD. The transmission frequency can e.g. be the normal European frequency of 50 Hz.
Another benefit with low transmission frequency is a strong reduction of the electrical skin effect of the transmission cable, i.e. better utilization of the cross section area of the cable. In practice transmission of high frequency electricity, say 100 Hz or more over ling distances, say 100 km or more, will become prohibitive due to the skin effect and the corresponding high resistance of the cable.
The influence of Ferranti effect and skin effect has of course to be calculated from case to case to assess whether they are acceptable or not for transmission at a given frequency. A demand exists for providing subsea electric power transmission systems that are beneficial with respect to the above mentioned problems.